What is JavaScript?

JavaScript is a scripting language widely used for client-side web development. It is also known by a number of other names including ECMAScript, and JScript (these are all the same thing but exist due to issues over the licensing of these names).

Books:

How/Why was YUI chosen for Moodle?

Since this decision was made, it's continued use has been questioned several times. Some of the main reasons for it's use include:

good documentation;

an active developer community;

code modularity;

a powerful loader system separating loading from execution and adding dependency resolution; and

sandboxing of code as standard.

Most other major libraries do not offer many of these major features including the powerful loader and dependency system which are particularly useful in the modular environment of Moodle.

What other JavaScript libraries are around?

jQuery

jQuery is another popular JavaScript library, used among others by Drupal, Joomla and WordPress. It is probably the most widely known JavaScript library at present.
It is possible to use jQuery in Moodle, but it is not recommended.

JavaScript written using jQuery will not be accepted within core Moodle code.

Can't we switch to jQuery in core?

Although jQuery is very popular with many developers and has stood out in recent years, there are a number of key infrastructure reasons which have the potential to cause major issues in a system like Moodle. As an example, some of these core infrastructure-type reasons include:

jQuery does not encourage sandboxing of code

jQuery does not have a decent modular system - plugins typically exist in the global $ namespace (and due to lack of sandboxing), this means that:

they can be modified after loading so other modules on the same page could modify a library function; and as a result

it is possible for jQuery to overwrite itself and break your page

jQuery does not have a loading system. You are typically expected to use the <script> tag which:

can be inefficient

means that your code is executed as soon as it loads;

which means that load order is often very significant

has mixed browser support with some of it's advanced features;

makes it difficult to make use of a combo loading system to load JS modules.

Whilst these reasons may not seem very important as they're just nuts and bolts JS issues, when considered in combination with the modularity of Moodle itself they make a lot more sense. Bearing in mind that Moodle encourages use of contributed plugins (blocks, modules, question types, etc), the above reasons make more sense:

a number of Moodle-specific YUI modules exist to encourage re-use of user interface components and functionality:

so we need dependency management;

and to ensure that load order of components does not matter;

and that multiple uses of the same JS code does not break former or subsequent uses by it's mere inclusion.

with the availability of contributed plugins for moodle we need to ensure that modules do not interfere with, or have negative effects on, one another;

with the potential number of non-core plugins, sandboxing of that code is important to prevent inadvertent issues caused by individual plugins (e.g. changing core library code to add features).

Moodle Docs

What does AJAX mean?

AJAX means "Asynchronous JavaScript and XML". It is a group of interrelated web development techniques used to create interactive web applications or rich Internet applications.
Although XML is included in the acronym, AJAX is not a single technology but a group of technologies. In reality with modern JS, this tends to be JSON which is much more lightweight and easy to produce in PHP than XML.